The present invention relates to a developing method and a developing apparatus of non-contact type for developing a latent image.
It is known, as disclosed in U.S. Pat. Nos. 4,292,387 and 4,395,476 for example, that one component, insulative and magnetic toner or non-magnetic toner is applied on a surface of a developer carrying member and is conveyed thereon to a developing station, where the thin layer of the toner is opposed with a clearance to a surface of a latent image bearing member. An alternating voltage is applied at this station so as to transport the toner from the developer carrying member to the latent image bearing member so as to develop the latent image, thus effecting the developing action without contacting the thin toner layer with the latent image bearing member.
However, the developing method or apparatus of this type involves the following problem. When an attempt is made to positively charge with certainty the toner particles on the developer carrying member surface to the intended polarity which is required for the movement of the toner particles in the developing station, the thickness of the toner layer on the developer carrying member surface becomes thin so that the density of the developed image is low, particularly in the case of a solid black part of the image. This will be explained further. When a latent image consisting of letters or characters is developed, the toner particles come to the letter portions not only from the area of the toner layer exactly opposed to the letter portions but also from the area in the neighborhood thereof by the alternating electric field, whereby a developed image of a sufficient density can be provided. When, on the other hand, a solid black image or a thick line image is developed, the amount of the thin layer toner particles on the developer carrying member surface tends to be insufficient, and the toner is concentrated at the edges of the latent image with the result that the developed image is formed with an insufficient amount of the toner particles.
When the magnetic toner particles each consisting of magnetic material and resin, it is difficult to reproduce an image in a bright non-black color since the magnetic toner particles contain the magnetic material, which is usually black in color. Therefore, for a color reproduction, non-magnetic toner mainly comprised of resin is used exclusively. However, the tendency of the lack of the toner in the solid black image is observed more remarkably when non-magnetic toner particles are used than when the magnetic toner particles are used. For those reasons, the above described problems are more significant when the color development is to be carried out than when monochromatic development. Particularly, in the case of a high quality development for a pictorial color reproduction, the above described edge effects and the lack of density in the solid image are significant problems.
Further, as one of non-contact type developing method, a proposal has been made wherein a developer including insulating toner particles mixed with conductive and magnetic carrier particles (hereinafter will be called "conductive carrier") is applied on the surface of the developer carrying member, the applied layer of the developer is opposed with a clearance to the surface of the latent image bearing member with an alternating voltage applied across the clearance so as to transit the toner particles from the layer to the latent image bearing member to develop the same.
This type of developing method involves the following drawbacks. In the developing action when the developer layer is opposed to the latent image bearing member with the alternating voltage applied across the clearance, the developer reciprocates across the clearance, resulting in the toner particles deposited to the image part (the part to which the toner should be deposited) of the latent image. It is required in order to obtain the desirable development that only the toner particles reciprocate or transit, but actually the conductive carrier particles also move to the latent image bearing member. As a result, the carrier particles can impinge on the toner particles already deposited on the image part of the latent image, which causes the deposited toner particles to scatter around. This disturbs the image to degrade the quality thereof. Additionally, when the conductive carrier particles reach the image part, they neutralize the electric charge of the latent image, thus reducing the image density. Furthermore, when the voltage is increased in an attempt to broaden the area of the voltage application, a spark discharge can take place across the clearance, which will destroy the latent image and additionally which can damage the latent image bearing surface. This occurs more easily when the resistance of the developer layer is low, so that the tolerable range of the AC voltage is very narrow. Moreover, if the conductive carrier particles are transferred to and deposited on the latent image bearing member surface, they are not transferred to a transfer material in the subsequent image transfer station, and therefore, they reach the cleaning station by being conveyed on the latent image bearing member. Then, the latent image bearing surface is "abraded" with the conductive carrier particles, which can damage the latent image bearing surface.
U.S. Pat. No. 4,450,220 proposes a non-contact developing method which uses two component developer with insulative carrier particles. U.S. patent application Ser. No. 632,887 proposes a non-contact developing method with the use of flat carrier particles. In those developing method, a developing magnet pole is opposed to the latent image bearing member in the developing station. Accordingly, the magnetic particles are formed into a magnetic brush in the developing station so that the thickness of the developer layer is not uniform there. This means that the distance or clearance between the developer layer and the latent image bearing member is not constant. As a result, when an alternating voltage is applied across the clearance, the strength of the electric field formed in the clearance is not uniform, which may lead to the occurrence of undesirable discharge.